Optical devices that are used in photonic integrated circuits (PICs), such as but not exclusively those formed in a silicon layer of a silicon-on-isolator (SOI) chip, are typically of planar geometry and operate best with light in a specific polarization state, and preferably in a fundamental mode. However, higher order waveguide modes could be excited in PIC waveguides due to various waveguide imperfections such as sidewall roughness, transitions between multimode and single mode regions, at fiber coupling, and other interactions of the optical signal with the geometry of the devices.
Generally, planar waveguides of the type conventionally used in a PIC can support modes of two orthogonal transverse polarization states, termed TE and TM, with the lower-order modes typically being better confined within the waveguide and characterized by a greater effective index than higher-order modes of the same polarization. The lowest-order modes of each polarization state are commonly referred to as the fundamental modes and denoted as TE0 and TM0, respectively.
In order to optimize PIC performance and reduce noise it is generally desired that light propagating in the PIC belongs to a fundamental mode of a particular polarization, most commonly TE0. While suppressing higher-order TE and TM modes may be effected by using sufficiently narrow waveguides that are often referred to as single-mode, such waveguides typically support the fundamental mode of both the TE and TM polarization. Hence, additional efforts may be needed to discriminate between the TE and TM light in a PIC and to suppress one of them.
One possible approach to solving this problem is to use a directional coupler or a multi-mode interference (MMI) coupler as a polarization splitter to split TE and TM modes in space and couple them into different waveguides. Directional couplers are however sensitive to variations in wavelength, which makes it difficult to achieve high TM/TE extinction ratio across a suitably wide wavelength range, for example across the entire C band. Another drawback of directional couplers is their low tolerance to fabrication inaccuracies. A drawback of using an MIMI coupler for splitting the TE and TM polarizations relates to the coupler length, which may have to be relatively big since the difference between effective indices of the TE0 and TM0 modes is typically small. Furthermore, an MMI coupler capable of splitting off the TM polarization may have a relatively high insertion loss for the TE mode. Proposed waveguide polarizers based on asymmetrical Mach-Zehnder Interferometer (MZI) and adiabatic couplers suffer from similar drawbacks, including a big device length and a relatively high insertion loss for the TE mode.
Accordingly, it may be understood that there may be significant problems and shortcomings associated with current solutions and technologies for providing a required level of suppression of light of undesired polarization and/or modes in photonic integrated circuits.